1. Technical Field
The present invention relates to a rotating circular die for producing an extruded food product and, in particular, to an improved extruder die assembly that incorporates an independently supported inner cone which can be easily adjusted during processing to vary the thickness of the extrudate and maintain extrudate uniformity.
2. Description of the Related Art
Extrusion devices used in the food industry introduce a dough into a device and convey it via, for example, a screw pump to an inlet where the substance is forced through an extruder die. The extruder die is employed to perform a variety of functions, including forming or shaping the extrudate, and compressing and reducing the cross sectional area of the extrudate.
One example of a commonly employed extruder die is a die comprising a linear extrusion exit channel. In operation, the extrudate is forced through the extruder passageway and through the linear die channel. The size of the channel may be adjusted by various means, such as using flexible upper and lower die lips to produce a range of possible extrudate thicknesses. In order to accommodate the linear die, however, the shape of the extruder passageway prior to the die is broadened in width, and flattened in height. Since a linear die has different flow path lengths, the passageway shape is varied to produce a pressure gradient in an attempt to achieve a uniform flow rate across the die. As a result, the flow rate can be lower at the edges than in the center of the channel, resulting in non-uniform extrudate. In addition, undesired thickness variations in the extrudate can be found in the die channel direction as well as in the direction of extrusion.
Another example of a commonly employed extruder die is a circular die comprising an outer ring component and an inner circle component that are fixed to form an annular extrusion exit channel. During processing, the extrudate is forced through the extruder passageway and through the circular channel between the stationary outer and inner components of the circular die. The travel distance for the extrudate in the passageway to any exit point on the circular die is consistent. Thus, the circular die avoids the thickness variation problems and extrudate flow problems inherent in the linear die. However, the circular die does not offer the flexibility of easily adjusting the die components to vary the thickness of the extrudate. To vary the thickness of the extrudate, either the outer or inner component of the die must be removed and replaced with another component that will provide the desired thickness setting. This lengthy process is cumbersome and also costly to manage, for production must be suspended until the replacement is installed.
FIGS. 1 and 2 show a circular die apparatus as disclosed in U.S. Pat. No. 4,680,191 to Budd et al. FIG. 1 shows a side view partially cut away of the die and extruder assembly, while FIG. 2 is a bottom plan view of the exit end of extruder. Corresponding reference numerals are used to represent corresponding elements unless otherwise indicated.
The circular die assembly of the '191 Patent comprises an outer facing 202 and inner facing 204, both of which can be rotated in a clockwise or counterclockwise direction. The outer and inner facings 202, 204 contain serrated edges that impart ridges and valleys on each side of the extrudate flow as it passes through a circular channel 206. Rotating one or both of the outer and inner facings 202, 204 during the extrusion process produces corrugations on one side of the extrudate at an angle to those on the other side. As a result, the corrugated end product achieves a different texture, flavor and appearance from non-corrugated products. However, the rotation speed of the die components cannot exceed a certain threshold wherein the serrated edges no longer impart distinct ridges and valleys on the surfaces of the extrudate. Thus the Budd device must rotate at a relatively low speed, such that the tangential velocity of the rotating portion of the device in contact with the extrudate is at most equal to the linear velocity of the extrudate upon exiting the die.
In contrast, “high-speed rotation,” meaning the rotational speed that produces a tangential velocity of the rotating portion of the device in contact with the extrudate that is greater than the linear velocity of the extrudate upon exiting the die, can be used as a means to create various different textures, flavors, and appearances of the end food product by further aligning the starch polymer chains in the extrudate. High-speed rotation allows the die to impart additional shearing stresses on the extrudate, thereby forcing the starch polymers into alignment in the direction of such shearing. Polymer alignment in the rotational direction of the extruder can be preferred over alignment only in the direction of the linear extrusion because bi-directional alignment provides for anticipation, control, and manipulation of the size, shape, and texture of the end food product. For example, extrudate or dry pellets subjected to high linear stress during extrusion expand during the frying process in directions perpendicular to the direction of the starch alignment caused by this linear stress. Stress induced on the extrudate by a rotating component provides for an additional direction of the starch alignment and various potential results produced by expansion of such a product. The result obtained by such expansion offers various possibilities over corrugated products of producing different sizes, shapes and textures of the end food product.
In addition, the higher the rotation speed, the more such rotation provides a means to prevent backup of extrudate by grinding out clogs in the die. Clogs of extrudate can form in the extruder passageway and become caught in the die channel. As a result, the normal flow of extrudate is hampered and a backup occurs in the passageway. To remove the back up, production may need to be interrupted in order to clear the clog from the extrudate pathway. Rotation allows the die to be self-clearing by breaking up and grinding out the clogs to prevent extrudate backup. Thus, rotation reduces plugging risk and provides a simpler and efficient solution to the costly alternatives of clearing clogs.
Although the Budd et al. device avoids the thickness variation problems inherent in the linear die for the same reasons as does the fixed circular die, the thickness of the extrudate exiting from die cannot be adjusted without replacing the inner or outer component with one with different dimensions. Replacing a component requires stopping production and dismantling the die assembly. This is a time-consuming operation and processing cannot resume until after the die is dismantled and reassembled with the new die component. Additionally, the die assembly must be dismantled to clear a clog or to clean the extruder and die assembly. Such cleaning, unclogging or component replacement times can result in reduced production and costly production down time.
FIG. 1 shows a cut-away side view of the '191 extrusion system which includes an extrudate passageway 102 and circular die assembly. The support for the inner facing 204 and its driving means 210 are housed within the extruder passageway 102. Placement of the inner facing components within the extruder can be problematic, however, since the inner facing components have additional surfaces to which food particles can adhere. This may result in additional or increased extruder cleaning periods and thus shortened production periods. In addition, the direct contact between the extrudate and the bearings and seals of the housings may result in sanitary and microbial spoilage issues.
Consequently, a need exists for a rotating circular die wherein the channel between the inner and outer components of the die can easily be altered to allow for various thicknesses of the extrudate, even during processing. In order to eliminate potential sanitary, reliability, and cleaning problems related to seal maintenance and extrudate contact with bearings or seals, the die components should not be housed within the extruder passageway. In addition, it would be desirable to produce a die with easily adjustable components that provide simple and efficient means to clear clogs and clean the extrudate pathway. Furthermore, a need exists for a die that provides for uniform thickness of the extrudate as it exits the die and also be capable of imparting rotational shearing stresses on the extrudate in order to provide for more control of biopolymer, such as starch, alignment. It would also be desirable to produce a die having means to heat and cool the die surfaces to vary the texture and appearance of the food product and also be able to monitor the consistency of the extrudate by either measuring the force the extrudate exerts on the die or the torque necessary to rotate the die and/or the extrudate temperature, or combinations thereof.